Propylene polymer resin with improved properties

ABSTRACT

The present invention relates to a process for the production of a propylene polymer in a multistage polymerisation process comprising the polymerisation of propylene in the presence of a catalyst in a first reaction zone comprising at least one slurry reactor to give a first polymerisation product, transferring said first product to a second reaction zone comprising at least one gas phase reactor and continuing polymerisation of propylene in the gas phase in the presence of said first polymerisation product which is characterised in that the temperature in both the slurry and the gas phase reactor is at least 75° C. and the product of at least one reactor is having an ethylene content in the range of 0.05 to 0.5 wt. %. Furthermore, the invention relates to a propylene polymer obtainable by the inventive process, to a propylene polymer which is characterised in that it comprises ethylene comonomer in an amount of 0.05 to 0.5 wt. %, is having a xylene solubles contents of 3.0 wt. % or less and is having a maximum in its temperature rising elution fractionation (TREF)-function at 120° C. or less, and to articles such as fibres, non-wovens, films and sheets comprising the inventive polymer.

This divisional application claims priority from and benefit of thefiling date of co-pending U.S. patent application Ser. No. 10/482,597filed May 27, 2004, which application is the U.S. National Phase ofInternational Application No. PCT/EP02/07083 filed Jun. 26, 2002(International Publication Number WO 03/002626), which claims priorityfrom and benefit of the filing date of European Patent Application No.01115470.5 filed Jun. 27, 2001. The specification of U.S. patentapplication Ser. No. 10/482,597 is hereby incorporated herein in itsentirety.

The present invention relates to a propylene polymer resin with improvedbonding properties, improved stretching properties and improvedthermoforming properties and a process for the production of such aresin. Furthermore, the present invention relates to a fibre comprisingsuch a resin and a non-woven fabric comprising such fibres, to a film,especially a bioriented film, comprising such a resin and to a sheet,especially for thermoforming, comprising such a resin.

Non-woven fabrics are porous sheets which are produced by fibres forminga web. In the production of non-woven fabrics from polypropylene fibresusually in a first step a propylene polymer composition comprisingfurther components, such as an antioxidant or an acid scavenger, is meltextruded at temperatures of above 200° C. Fibres are then spun bypassing the melt through a spinnerette, quenching and winding up theproduced fibres. Optionally, after quenching one or more stretchingsteps are applied to the fibres. Such spinning processes today arecarried out at high speed in the order to 1000 nm/min up to 4000 m/minbut still an increase of line speed and output is desirable due toeconomic reasons.

Non-woven fabrics are then produced from the polypropylene fibres eitherin the form of filaments or stapled fibres by forming a web, usuallyfollowed by a final bonding step wherein the fibres are bound togetherto increase the strength of the web. This bonding step usually isperformed by applying heat and pressure to the web by passing the webthrough a calendar. When staple fibres are used the web forming usuallycomprises a carding step.

The bonding process affecting the fibre surface happens within a veryshort time and temperature window. Thus, in the production of thermalbonded non-woven fabrics the bonding step is limiting the maximum linespeed. Consequently, an improvement of the bonding properties of thefibres which results in an improvement of the bonding step, e.g. withrespect to the maximum line speed, is desirable.

Further, also the mechanical properties of the non-woven fabric dependon the bonding properties of the fibres and thus, better bondingproperties of the fibres lead to improved mechanical properties, inparticular mechanical strength, of the non-woven fabric.

It is therefore an object of the present invention to provide apropylene polymer for the production of polypropylene fibres withimproved bonding properties.

It is known that by broadening the molecular weight distribution of apropylene polymer small improvements of the bonding properties of fibrescomprising such polymer can be obtained. Further, it is known thathigher crystallinity of a polymer used for the production of fibresnegatively effects the bonding properties.

U.S. Pat. No. 5,281,378 describes an improvement in bonding propertiesof fibres due to a combination of an optimised molecular weightdistribution of the polymer and a delayed quenching during fibrespinning.

In the production of films, the use of polypropylene resins is wellknown. The mechanical and optical properties of such a film which arecrucial for its applicability are mainly determined by the properties ofthe resin used for film production. In particular, it is desired thatthe film shows good stretching behaviour, especially if the film isbiaxially oriented after its production e.g. by casting.

It is therefore a further object of the present invention to provide apropylene polymer for the production of a polypropylene film withimproved mechanical, especially stretching, properties.

It is further known to use sheets comprising polypropylene resins forthe production of articles by thermoforming processes. In theseprocesses the thermoforming properties of the sheets are mainlydetermined by the resin used for the sheet production.

It is therefore a further object of the present invention to provide, apropylene polymer for the production of a polypropylene sheet withimproved thermoforming properties.

The present invention is based on the finding that a propylene polymerachieving the above-given objects can be obtained by a process in whichthe polymerisation of propylene is carried out at elevated temperatureand in the presence of small amounts of ethylene.

The present invention therefore provides a process for the production ofa propylene polymer in a multistage polymerisation process comprisingthe polymerisation of propylene in the presence of a catalyst in a firstreaction zone comprising at least one slurry reactor to give a firstpolymerisation product, transferring said first product to a secondreaction zone comprising at least one gas phase reactor and continuingpolymerisation of propylene in the gas phase in the presence of saidfirst polymerisation product characterised in that the temperature inboth the slurry and the gas phase reactor is at least 75° C. and theproduct of at least one reactor is having an ethylene content in therange of 0.05 to 0.5 wt. %.

In a preferred embodiment there is a temperature difference between theslurry and the gas phase reactors so that in the gas phase reactor thetemperature is higher than in the slurry reactor.

Preferably, the temperature in the gas phase reactor is at least 3° C.and more preferably at least 5° C. higher than in the slurry reactor.

Preferably, the temperature in both slurry and gas phase reactor is atleast 80° C.

In a particularly preferred embodiment the temperature in the slurryreactor is around 80° C. and the temperature in the gas phase reactor isabout 85° C.

Further, the present invention provides a propylene polymercharacterised in that it comprises ethylene comonomer in an amount of0.05 to 0.5 wt. %, is having a xylene soluble (XS) content of 3.0 wt. %or less and is having a maximum in its temperature rising elutionfractionation (TREF)-function at 120° C. or less.

The “TREF-function” of the polymer shows the eluted weight fraction ofthe polymer plotted as a continous function of the elution temperatureaccording to the procedure as given in the Examples section.

The inventive process and the inventive polymer allow the production ofpolypropylene fibres having improved bonding properties so thatnon-woven fabrics comprising such fibres can be produced with higherbonding speed and/or with improved mechanical properties, in particularmechanical strength. The present invention also relates to these fibres.

The inventive process and the inventive polymer allow the production ofpolypropylene films having improved mechanical, especially stretching,properties so that e.g. biaxially oriented films with improvedproperties can be produced. The present invention also relates to thesefilms.

Further, the inventive process and the inventive polymer allow theproduction of polypropylene sheets having improved thermoformingproperties so that the production of articles by thermoforming isimproved; The present invention also relates to these sheets.

Furthermore, the present invention relates to a process for theproduction of a non-woven fabric which is characterised in that itcomprises forming a web comprising the inventive fibres and bonding theweb, as well as to a non-woven fabric comprising the inventive fibres.

Such webs and hence non-wovens may be produced either by staple fibres,i.e. fibres which have been stapled after their production, or filamentsas e.g. continous filaments. The term “fibres” as used herein isintended to cover both staple fibres and filaments.

The bonding of the fibres in the fibrous web upon which the non-woven isbased gives strength to the web and influences its properties ingeneral. One widely used method of bonding such fibrous webs is by meansof heat. Manufacturing methods for non-wovens are described in a varietyof publications, for example “The Nonwovens Handbook” (The Associationof the Nonwoven Industry, 1988) and the “Encyclopaedia of PolymerScience and Engineering”, Volume 10, Nonwoven fabrics (John Wiley andSons, 1987).

The properties of polypropylene are dependent on the degree ofcrystallinity and lamellae size distribution. It is known that highyield Ziegler-Natta catalyst has a tendency to concentrate stereodefectsinto the low molar mass chains leading to a broad tacticity distributionand intermolecular heterogeneity of polypropylene (L. Paukkeri et. al.,Polymer, 1993, 34, 2488–2494) leading also to an increased amount ofxylene solubles.

From a material point of view the distribution of stereodefects is amore important parameter than the total isotacticity. Good (random)defect distribution means more even (narrower) isotactic sequence lengthdistribution and lower amount of xylene solubles.

By changing polymerisation conditions (in particular temperature) it ispossible to control—to some extent—the relative amount and averagelength of isotactic and atactic sequences. The random distribution ofstereodefects corresponds to a lower average length of perfectlyisotactic sequences compared to the case in which such stereodefects aresegregated in isotactoid blocks.

The average length of isotactic sequences can also be influenced by thecontrolled incorporation of co-monomeric units acting as stericaldefects in the polymer chain.

For the determination of the isotacticity distribution, the TREF methodcan be applied. TREF is a common method to fractionate polyolefinsaccording to their solubility differences. The solubility of apolypropylene polymer chain is influenced only by the concentration ofsterical defects. It has been demonstrated for polypropylene that TREFfractograms qualitatively reflect the distribution of isotacticity. Theaverage length of isotactic chains increases almost linearly withincreasing elution temperature (P. Ville et al., Polymer 42 (2001)1953–1967). The results further showed that TREF does not strictlyfractionate polypropylene according to tacticity but according to thelongest crystallisable sequences in the chain.

According to the invention it has been found that the inventive processprovides for an even ethylene comonomer distribution in the inventivepropylene polymer. These ethylene comonomers act as sterical defects andhence interrupt the sequence of isotactic propylene monomers. Thus, byan even distribution of the ethylene comonomers an even distribution ofsterical defects is obtained, i.e. it is possible by the inventiveprocess to tailor the defect distribution and hence the isotacticitydistribution of the polypropylene polymer.

As a consequence, a narrower sequence length distribution of theisotactic propylene monomer sequences is obtained. Hence, on the onehand, a lower amount of xylene solubles which predominantly comprisepolymer chains with small isotactic propylene sequence lengths, and, onthe other hand, a lower amount of polymer chains with long isotacticpropylene sequence lengths are obtained. This, in turn, leads to animprovement of the bonding properties of fibres comprising suchpropylene polymer due to e.g. an even melting point of the polymer andtailored isotacticity.

The bonding properties of the fibres are measured via the measurement ofthe bonding index (BI) of the produced non-woven fabric which is definedas the square root of the product of the bonding strength in the machinedirection (MD) and in the cross-direction (CD) quoted as N/5 cm:BI=√{square root over (CD·MD)}

As the strength in the machine direction (parallel to the movement ofthe web/non-woven) is often different from the cross directionalstrength, the bonding index is a function of both of these. In optimalinstances, the ratio between the MD strength and the CD strength isaround unity.

Further, the bonding window is defined the temperature interval in whicha bonding index in the non-woven is obtained which differs from themaximum bonding index BI_(max) obtained at optimum bonding temperatureby not more than 15%. In case of a typical good quality non-woven foruse e.g. in hygienic absorbent products this corresponds to a differencein the bonding index of about 3 N/5 cm compared to BI_(max).

A broad bonding window gives the producer of non-woven fabrics a betterpossibility of obtaining a uniform product even when using a calenderingsystem with temperature variation over the calender surface or whenusing a higher bonding speed or lower bonding temperature. This is aconsiderable advantage for the non-woven producer.

“Slurry reactor” designates any reactor such as a continuous or simplebatch stirred tank reactor or loop reactor operating in bulk or slurry,including supercritical conditions, in which the polymer forms aparticulate form.

The inventive process is a multistage processes for the production ofpropylene polymers. Such processes are described in EP 0 887 379, forexample. The contents of this document is herein included by reference.

As catalyst, all kind of chemical compounds suitable for thepolymerisation of propylene can be used as e.g. Ziegler-Natta, andsingle-site catalysts such as metallocene catalysts. If single-sitecatalysts are used, those described in WO 95/12622 and WO 00/34341 arepreferred.

In a preferred embodiment, a Ziegler-Natta-type catalyst systemcomprising a catalyst component, a co-catalyst component and an externalelectron donor is used. Such catalyst systems are described in, forexample, U.S. Pat. No. 5,234,879, WO 92/19653, WO 92/19658 and WO99/33843.

The used external donors are preferably silane based donors, especiallydi-cyclopentyldimethoxysilane (donor D).

Optionally, the main polymerisation stages may be preceded by aprepolymerisation, in which up to 10% by weight, preferably 0.1–10% byweight and most preferred 0.5 to 5% by weight of the total amount of thepolymer is produced.

In a preferred embodiment of the inventive process both the productproduced in the slurry and that produced in the gas phase reactor ishaving an ethylene content in the range of 0.05 to 0.5 wt. %.

Further preferred, the ethylene content of the product produced in atleast one of the reactors is in the range of 0.15 to 0.4 wt. % and mostpreserved is around 0.3 wt. %. These preferred and most preferred valuesalso apply for the ethylene content of both products in the preferredprocess embodiment in which in both slurry and gas phase reactor aproduct comprising ethylene is produced.

Further preferred, in the inventive process the slurry reactor is a bulkreactor. “Bulk” means a polymerisation in an reaction medium comprisingat least 60 wt. % monomer.

Preferably, the bulk reactor is a loop reactor.

Preferably, in the inventive process the production split between theslurry reactor and the gas phase reactor is from 70:30 to 40:60, morepreferred from 60:40 to 50:50.

It is further preferred that the reaction temperature in both reactorsis 100° C. or less, more preferably 95° C. or less.

The inventive propylene polymer preferably is produced in a process,including the preferred embodiments, as described above.

As outlined above, the present invention provides a propylene polymercharacterised in that it comprises ethylene comonomer in an amount of0.05 to 0.5 wt. %, is having a xylene soluble (XS) contents of 3.0 wt. %or less and is having a maximum in its temperature rising elutionfractionation (TREF)-function at 120° C. or less.

The term “xylene solubles” (XS) designates the fraction of the polymersoluble in xylene, determined to the procedure as outlined in theExamples section.

In a preferred embodiment the inventive propylene polymer ischaracterised in that the xylene solubles content is 2.5 wt. % or less.

It is further preferred that the propylene polymer is having a maximumin its temperature rising elution fractionation (TREF)-function at 118°C. or less and still more preferred at 115° C. or less.

Further preferred the inventive propylene polymer comprises ethylenecomonomer in an amount of 0.15 to 0.4 wt. %, still more preferred in anamount of around 0.3 wt. %.

Preferably, the inventive propylene copolymer has a melt flow rate MFR₂of 1 to 50 g/10 min, more preferred of 5 to 20 g/10 min and mostpreferred of 10 to 16 g/10 min measured according to ISO 1133 (230° C.,2.16 kg load).

The molecular weight distribution (MWD) of the polymer materialpreferably is in the range of 2 to 7, more preferably 4 to 6.

The propylene polymer obtained according to the inventive processusually has a high degree of isotacticity.

Preferably, the fibres comprising the inventive propylene polymer have abonding index of 20 or more, more preferred of 21.5 or more and mostpreferred of 23 or more in a non-woven fabric.

Further preferred, the polymer has a degree of crystallinity from 40 to60%, more preferred from 48 to 60% and most preferred from 50 to 57%.Crystallinity is determined in accordance with two ISO 11357-03 and ascientific background is given in A. P. Grey, Thermal Chimica Acta 1970,1, page 563.

In the production of non-woven fabrics comprising the inventive fibresthe bonding process preferably is performed at a speed of at least 150m/min, more preferably of at least 200 m/min and most preferably atleast 250 m/min.

Bonding preferably is performed by thermal bonding, e.g. calendarbonding or hot air bonding, infrared bonding or ultrasound bonding.Further preferred, bonding is performed by thermal bonding preferably ina calendar.

In the following, a preferred embodiment of the inventive process andcopolymer will be illustrated by means of examples with reference to theenclosed figures.

FIG. 1 shows a TREF fractogram of the final polymer (GPR) according tothe invention (Example 1) and of that produced in the loop reactor only.

FIG. 2 shows a TREF fractogram of the polymer according to the invention(Example 1) and of the comparative polymer (Example 2).

FIG. 3 shows the TREF-function of the final polymer (GPR) according tothe invention (Example 1) and of that produced in the loop reactor onlycalculated from the results given in FIG. 1.

FIG. 4 shows the TREF-function of the polymer according to the invention(Example 1) and of the comparative polymer (Example 2) calculated fromthe results given in FIG. 2.

EXAMPLES

1) Measuring Methods

a) TREF-Method:

Fractionation of the polypropylene samples was achieved by usinganalytical TREF. The TREF profiles were generated using a home madeinstrument, which is similar to a published design (Wild, L., TrendsPolym Sci. 1993, 1, 50).

The sample was dissolved in xylene (2–4 mg/ml) at 130° C. and injectedinto the column at 130° C., and the latter was then cooled to 20° C. ata rate of 1.5 K/h. The column was subsequently eluted with1,2,4-trichlorobenzene (TCB) at a flow rate of 0.5 ml/min while thetemperature was increased from 20° C. to 130° C. over 4.5 h. The output,detected with an i.r. detector operating at a wavelength of 3.41 μm, waspresented as a fractogram normalised to constant area.

b) Xylene Solubles (XS):

For the determination of the xylene solubles fraction, 2.0 g of polymeris desolved in 250 ml p-xylene at 135° C. under agitation. After 30±2min the solution is allowed to cool for 15 min at ambient temperatureand then allowed to settle for 30 min at 25±0.5° C. The solution isfiltered with filter paper into two 100 ml flasks.

The solution from the first 100 ml vessel is evaporated in nitrogen flowand the residue is dried under vacuum at 90° C. until constant weight isreached. The xylene soluble fraction is calculated using the followingequation:XS %=(100·m ₁ ·v ₀)/(m ₀ ·v ₁)wherein

-   m₀=initial polymer amount (g),-   m₁=weight of residue (g),-   v₀=initial volume (ml),-   v₁=volume of analysed sample (ml).    c) M_(w)/M_(n)

M_(w)/M_(n) was determined using gel permeation chromatography (GPC) at130° C. As an eluent, 1,2,4-trichlorobenzene (TCB) was used.

d) Melt Flow Rate (MFR)

MFR₂ was measured according to ISO 1133 at 230° C. and a load of 2.16kg.

e) Thermal Properties

Melting temperature T_(m), crystallisation temperature T_(cr), and thedegree of crystallinity were measured with a Mettler TA820 differentialscanning calorimetry (DSC) on 3±0.5 mg samples. Both crystallisation andmelting curves were obtained during 10° C./min cooling and heating scansbetween 30° C. and 225° C.

Melting and crystallisation temperatures were taken as the peaks ofendotherms and exotherms. The degree of crystallinity was calculated bycomparison with heat of fusion of a perfectly crystalline polypropylene,i.e. 209 J/g.

2) Propylene Polymer Production

Example 1

Invention

Continous multistage process was used to produce propylene (co)polymer.The process comprised a prepolymerisation step, a first polymerisationstage carried out in a loop reactor and a second polymerisation stagecarried out in a fluidized bed gas phase reactor.

As a catalyst, a highly active, stereospecific transesterifiedMgCl₂-supported Ziegler-Natta catalyst prepared according to U.S. Pat.No. 5,234,879 at a titanization temperature of 135° C. was used. Thecatalyst was contacted with the cocatalyst (triethylaluminium, TEAL) andthe external donor which was dicyclopentyl dimethoxysilane with a Al/Tiratio of 200 and a Al/D ratio of 10, to yield the catalyst system.

The catalyst system and propylene were fed into the prepolymerisationreactor which was operated at 30° C. The prepolymerised catalyst wasused in the subsequent polymerisation reactors.

Propylene, ethylene and hydrogen and the prepolymerised catalyst werefed into the loop reactor which was operated as bulk reactor at 80° C.at a pressure of 55 bar.

Then, the polymer slurry stream was fed from the loop reactor into thegas phase reactor which was operated at 85° C. and a pressure of 20 bar.More propylene, ethylene and hydrogen were fed into the gas phasereactor to control the desired properties of the final polymer.

The production split between loop and gas phase reactor was 60:40.

The polymer was melt homogenised and additivated as normal with 1300 ppmof antioxidants and UV stabiliser.

Example 2

Comparative

The same production procedure as in Example 1 was used except theprocess comprised two loop reactors instead of a loop and a gas phasereactor and no ethylene was fed to the process. Operation temperature inboth reactors was 70° C. The catalyst system used was the same as inexample 1 except that as an external donor cyclohexylmethyldimethoxysilane was used.

Additivation of the comparative polymer was the same as in Example 1.

3) Spinning Process/Web Formation

The polypropylene polymers produced according to 2) were used for fibreand subsequent non-woven production.

An ESL-pilot conventional spinning line was used to produce staplefibres. The spinning temperatures were in the range of 270–285° C.During spinning, the MFR₂ of the propylene fibres increased to approx.40 g/10 min due to thermal degradation.

The fibres had a fineness of 2.2 dTex. The fibres were texturised to alevel of about 12 crimps/cm and cut to 40 mm staple fibres.

Non-woven fabrics were produced using a Hergeth monolayer/Kusterscalender having a width of 600 mm. The winder speed of the process linewas 100 m/min. The produced web was a web having a weight of 20 gram persquare meter.

4) Results

The results of the examples performed are shown in FIG. 1 to 4 and Tab.2 which show the results of the TREF analysis of the polypropylenepolymers and Tab. 1 which shows further properties of the polymers andthe produced fibres/non-wovens.

TABLE 1 Polymer and fibre/non-woven properties Sample Unit InventionComparative Ethylene wt. % 0.17 0.0 MFR₂ g/10 min 13.0 13.6 XS wt. % 1.63.6 T_(m) of PP ° C. 162 162.5 crystallinity % 53.5 47.6 T_(er) of PP °C. 118.4 116 M_(n) 51100 55350 M_(w) 264000 259000 M_(z) 682500 665500M_(w)/M_(n) 5.2 4.7 Spinning speed 1800 1800 Bonding index 23.6 19.4

TABLE 2 Results of TREF analysis Invention Comparative TREF Unit loopgas phase (final) (final) <103° C. wt. % 1.7 3.2 0.1 <112° C. wt. % 1419 6 <118° C. wt. % 47 61 26

1. A propylene polymer characterised in that it comprises ethylenecomonomer in an amount of 0.05 to 0.5 wt. %, and has a xylene solublescontents of 3.0 wt. % or less and has a maximum in its temperaturerising elution fractionation (TREF)-function at 120° C. or less.
 2. Apropylene polymer according to claim 1 characterised in that the xylenesolubles contents is 2.5 wt. % or less.
 3. A propylene polymer accordingto claim 1 characterised in that it has a maximum in its temperaturerising elution fractionation (TREF)-function at 118° C. or less.
 4. Apropylene polymer according to claim 1 characterised in that itcomprises ethylene comonomer in an amount of 0.15 to 0.4 wt. %.
 5. Apropylene polymer according to claim 1 characterised in that it has amaximum in its temperature rising elution fractionation (TREF)-functionat 115° C. or less.
 6. The propylene polymer of claim 1 wherein thepolymer has an even distribution of ethylene comonomer.
 7. The propylenepolymer of claim 1 wherein the polymer has an even distribution ofsterical defects.
 8. The propylene polymer of claim 1 wherein amolecular weight distribution of the polymer is 2 to
 7. 9. The propylenepolymer of claim 1 wherein a melt flow rate of the polymer is 1 to 50g/10 mm.
 10. The propylene polymer of claim 1 wherein a degree ofcrystallinity of the polymer is 40 to 60%.
 11. A fiber comprising apropylene polymer defined in claim 1 having a bonding index of 20 ormore.
 12. A propylene polymer characterised in that it comprisesethylene comonomer in an amount of 0.05 to 0.5 wt. %, and has a xylenesolubles contents of 3.0 wt. % or less, a maximum in its temperaturerising elution fractionation (TREF)-function at 120° C. or less, and adegree of crystallinity of 50 to 60%.